Polypropylene resins are widely used mainly as injection molding materials in the fields of industrial components and containers represented by e.g. components of electric home appliances, automobile components, components for household equipments and food containers, and the quantity of their use has been steadily increasing due to a remarkable progress in the catalyst/polymerization techniques and combined techniques in recent years.
In a case where a polypropylene resin is used for the purpose of electromagnetic wave-shielding, electrostatic coating or prevention of static charge among such applications, it is necessary to impart electrical conductivity, since the resin itself is naturally electrically insulating. In such a case, it is known to provide electrical conductivity by e.g. incorporating an electroconductive material represented by e.g. carbon black or by coating the surface of a molded product with a paste-form conductive material. Among such methods, coating with a conductive material requires a solvent to make a paste, and a coating step is required, such being not efficient. Therefore, a method of making a resin composition itself to be electroconductive by kneading a conductive material such as carbon black into a polypropylene resin, is adopted as a more efficient method.
The conductive material to be incorporated to the polypropylene resin includes, in addition to carbon back, one in a particle form such as graphite, titanium oxide or metal powder, and one in a fiber form such as carbon fibers, metal fibers, or conductive whiskers. However, every one of such conductive materials has a specific gravity larger than the polypropylene and tends to increase the specific gravity of the composition by its incorporation. Accordingly, with a view to minimizing the weight, it is desired to provide desired electrical conductivity by minimizing the amount of the conductive material to be incorporated. Further, one in a fiber form is not preferred, since the appearance of the composition tends to deteriorate by its incorporation, or the fibers are likely to break during repeated use (recycling), whereby the electrical conductivity is likely to be impaired.
For such reasons, it is presently most preferred to employ carbon black as a conductive material. However, by the incorporation of conductive carbon black, the flowability of the polypropylene resin composition is lowered substantially. Accordingly, especially in its application to injection molding, improvement of the moldability is desired. Further, it is generally known that among conductive carbon black, Ketjen black having activation treatment applied thereto, is most superior in the electrical conductivity. However, Ketjen black is poor in dispersibility into a polypropylene resin, and although the electrical conductivity will be improved, there has been a problem that the tensile elongation tends to be inferior due to the poor dispersibility.
Therefore, it has been desired to develop a carbon black-incorporated electroconductive polypropylene resin composition excellent in the balance of specific gravity, electroconductivity, flowability and tensile elongation.